1. Field of the Invention
The present application relates to a compact swing arm structure for a shaft-driven vehicle, such as a motorcycle, and a to a vehicle incorporating such swing arm structure.
Shaft-driven motorcycles have a pivot shaft and a drive shaft, respectively disposed such that axes thereof cross each other in a top plan view. Vertically, these axes may be orthogonal to each other or offset from each other, not intersecting. This type of swing arm is known as a non-intersecting swing arm. In an embodiment of the present invention, the pivot shaft supports a front end of a swing arm, while the drive shaft is housed longitudinally in a hollow portion inside the swing arm.
2. Background Art
German Patent No. 2616101 shows a motorcycle having the non-intersecting type of swing arm. This swing arm or rocker has a tubular member that accommodates a drive shaft. The drive shaft includes a universal joint that appears to have its center of pivotal motion disposed directly above the center of pivotal motion of the swing arm.
With the orthogonal type, a commonly found universal joint may be sufficient for use as a joint between the drive shaft and an engine output shaft. The pivot shaft is, however, divided by the universal joint. To enhance suspension stiffness, therefore, it becomes necessary to make a bearing span of the pivot shaft long or enhance stiffness at a bearing portion of the pivot shaft. Having a long bearing span, however, results in a wider vehicle body width and narrower bank angle. Enhancing stiffness at the bearing portion, on the other hand, leads to an increased swing arm weight. In either case, both of these swing arm arrangements contribute to a larger size and a heavier weight of the vehicle and/or frame.
Adoption of the non-intersect type swing arm, on the other hand, does not result in the pivot shaft being divided by the universal joint. Accordingly, an ample bearing span can be obtained without having to enlarge the swing arm width. Use of a constant-velocity universal joint, however, becomes necessary for the joint. Further, the joint and the pivot shaft disposed by being vertically offset from each other results in a larger diameter at a front end of the swing arm, larger by a height of the joint and the pivot shaft. As a result, the swing arm becomes larger and heavier, if not equivalent to the orthogonal swing arm type. There is also a need for a lighter unsprung weight and, for that matter, for a lighter swing arm.
It is therefore an object of the present invention to make a vehicle width compact by adopting an improved non-intersect type swing arm, thereby achieving reduction in weight.